In lithographic printing, lithographic ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and lithographic ink is applied, the hydrophilic regions retain the water and repel the lithographic ink and the lithographic ink receptive regions accept the lithographic ink and repel the water. The lithographic ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the lithographic ink can be first transferred to an intermediate blanket that in turn is used to transfer the lithographic ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the element is considered as positive-working. Conversely, if the non-exposed regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are lithographic ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel lithographic ink.
“Laser direct imaging” methods (LDI) are used to directly form an offset printing plate or printing circuit board using digital data from a computer, and provide numerous advantages over the previous processes using masking photographic films. There has been considerable development in this field from more efficient lasers, improved imageable compositions and components thereof.
Various radiation-sensitive compositions are used in negative-working lithographic printing plate precursors as described for example in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), U.S. Pat. No. 6,727,281 (Tao et al.), U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Pat. No. 6,919,411 (Fujimako et al.), U.S. Pat. No. 8,137,896 (Patel et al.), and U.S. Pat. No. 7,429,445 (Munnelly et al.), U.S. Patent Application Publications 2002/0168494 (Nagata et al.), 2003/0118939 (West et al.), and EP Publications 1,079,276A2 (Lifka et al.) and 1,449,650A2 (Goto et al.). Many other publications provide details about such negative-working radiation-sensitive compositions comprising necessary imaging chemistry dispersed within suitable polymeric binders.
After imaging, the negative-working lithographic printing plate precursors are developed (processed) to remove the non-imaged (non-exposed) regions of the imageable layer.
In recent years, offset lithographic printing plates have had to meet ever growing demands for resistance to solvents and common press room chemicals such as plate cleansers or blanket wash solutions and alcohol substitutes in the fountain solution. In particular, when printing with UV-curing inks, where washing solutions with a high ester, ether, or ketone content are used, the chemical resistance of conventional lithographic printing plates, especially negative-working lithographic printing plates, is no longer sufficient unless they are subjected to special stabilizing processes.
It is also desired that the lithographic printing plates exhibit high abrasion resistance along with the improved chemical resistance. It is not always possible to achieve both properties with the same chemical compositions. What may improve abrasion resistance may not affect chemical resistance. In addition, the features that may improve chemical resistance may diminish imaging sensitivity. The desired chemical resistance can also be increased by baking the imaged and processed lithographic printing plates. However, not all imaging formulations are suitably baked to provide chemical resistance.
U.S. Pat. No. 8,137,891 (Jarek et al.) describes unique polymeric binders that can be used in both negative-working and positive-working lithographic printing plate precursors to improve bakeability and thus chemical resistance. However, there is a need to increase the photospeed to meet the needs of lithographic printing customers.
Thus, while previous research and development of chemically resistant polymer binders for lithographic printing plate precursors has provided some improvement, the technology has room for further improvement. While known polymeric binders have provided improved chemical resistance, there is a desire to provide even more improvement in bakeability so as to provide higher processing speeds, good shelf life, and run length.